To meet increasing demands and decreasing margins for paper products, pulp producers look for ways to maximize pulp production. One result is that pulp digesters become overloaded, i.e., they are run to their production limits. At these limits, the digester typically cooks pulp at very high temperatures with low retention times. Introducing cool white liquor to the process reduces the cooking temperature and negatively impacts the production level.
For example, in single-vessel vapor phase digesters, and particularly those in production lines that lack internal liquor impregnation or otherwise are not fed with internally impregnated chips, the lack of impregnation leads to undesirably high reject levels, e.g., up to 10% by weight at 17 Kappa number. Even in lines having internal liquor impregnation, as noted above, the introduction of cool white liquor from the impregnation into the digester reduces the cooking temperature and negatively impacts the production level. Heating the impregnated chips beforehand exacts a high energy cost, however.
These and other problems are solved by embodiments of the present invention, one of which provides a system and method to deliver impregnated lignocellulosic material to the digester while maintaining the feed at an elevated temperature.